Tyre pressure formula for older bias ply tyres now with fitted with radials

[rodneybeauchamp]

Hi all,

 

Am running two 1963 Buicks that originally were shod with bias ply tyres. However both now happily run modern radials on different diameter rims.

 

Is there a formula I can use to calculate the correct tyre pressure I should be running in each Buick ( Riviera and Skylark convertible ) ?

 

Both cars weigh differently and on the forums I see many guesses but surely there must be an exacting method to calculate correct pressures.

 

Trying to feel it by the seat of your pants method to me is not scientific so lnterested to know if there is a formula. EG Weight of Car x Tyre diameter / tyre width x 0.7654321

 

Many thanks
Rodney 😀😀😀😀😀😀😀😀

[JACK M]

Grab your thermo gun and take a long drive.

Have a note pad with you.

Pull over when you think your tires are up to temp.

Take a temp reading in three places, (inner, center and outer) on all four tires. (I usually draw an overhead view with lines for these numbers)

You may have to adjust the air pressure to get these numbers to be fairly equal.

 

This is how we get air pressure (and camber) suggestions for the dirt cars.

Should be a good trick for a driver car as well.

[Mikefit]

Very   well explained Rodney. 

[Bloo]

19 hours ago, rodneybeauchamp said:

Both cars weigh differently and on the forums I see many guesses but surely there must be an exacting method to calculate correct pressures.

No, there isn't.

 

The best you will do with a formula is to check what load range the tires have listed, and attempt to look it up. There may be tables of load vs pressure. Also, P-metric is not the same as metric. For instance P205/75-15 is not the same as 205/75-15. The size is the same, at least in theory, but the way they look at load ratings is different. Without going down the rabbit hole again, the way I remember it is there are load tables for metric tires for a given load range (reflects reality) but for P-metric tires, they are supposed to support some given load across a range of pressures (does not reflect reality). In theory you could look at some table if one exists, weigh the corners of your car, add for passengers and luggage, and calculate what pressures to run front and rear. Maybe even side to side. If you do this, the numbers you get will be in my opinion too low. It will be the softest squishiest setting that can in theory handle the load. If a tire loses ANY air at all, you will be overloaded instantly. Most tire failures are due to underinflation and overloading. When in doubt, more air is always better.

 

I tell you all this with the assumption that you are looking at tires with US market markings, yet knowing full well your location is listed as Australia. It is a deep enough rabbit hole with the tires available in the US, but you could have tire markings in Australia that I have never seen before.

 

@JACK M's method is the best way if you need accuracy. That is how it is done in road racing on pavement tracks, too, although I don't think you can be fast enough with your measurements on pavement to do it the way @JACK M suggested. In my opinion you need a friend to help take measurements. you have to come in from the "track" or whatever your test road is with the tires hot, pull off and instantly have your friend check. One tire per test. They cool off way too fast. In my opinion, you don't have time to get out of the car. We used tire pyrometers, because there weren't thermal guns yet. The pyrometer is slower to use, so the gun works to your advantage that way, but the surface probably cools faster too, and since you are measuring the surface rather than poking in with a pyrometer, that probably works against you. The biggest problem we always had was measuring fast enough. Look for hotter in the center if the pressure is too high, and colder in the center if the pressure is too low. If it is hotter on the outside edge than the inside, you need more negative camber. If it is hotter on the inside edge than the outside you need less. Radials are less sensitive to pressure change than bias tires, because the radials have a reinforcement belt under the tread, and it takes more pressure change to make them bulge in the center. I have no experience dirt racing, so I'll defer to @JACK M on that. It sounds like almost exactly the same method.

 

In the 80s, here in Washington State USA, we didn't use corrosive materials on the roads and 20 and 30 year old (or more) cars were everywhere and got driven year round. These all came with bias tires originally, and almost all of them had radials. Maximum sidewall pressure was the rule, and in the 80s that was either 32 or 35psi. If someone liked it squishy, maybe he got 30psi instead. Never less than that. A few tires were rated 44psi but you hardly ever saw that. Those probably got 35psi like everything else, maybe 40psi if you like it a little crisper (I do). If a pickup (3/4 ton or larger) or trailer tire was rated 50psi max, it got 50psi. After all, It was expected to carry a load. Most of those were still bias at that time, especially trailers, but a few used radials. I still run 35psi in old American rear wheel drive cars with radials installed. It has never been a problem, nor caused uneven tire wear.

 

 

Edited June 13, 2022 by Bloo (see edit history)

[Larry Schramm]

I just use the maximum tire pressure that is put on the side of the tire.  This also gives you the most load carrying capability of the tire. 

 

Only downside is that you might get a little harsher ride, but it will give your better fuel economy.  

 

Last set of tires that I had on my Silverado I got 125,000 miles on the tires.  They were Michelin LT tires.

[EmTee]

I recently installed a new set of tube-type reproduction bias-ply tires (7.00 - 15) on my '38 Century.  The shop manual calls for 23 psi front and 28 psi rear (cold).  I don't really understand why the front pressure would be so much lower, particularly with that big, cast-iron straight-8 up front...  Believing that more pressure is better (safer) I started with 34 psi front and 30 psi rear.  I ran this way for a couple hundred miles or so checking tire temps with my IR thermometer upon return.  Car seemed to handle well and the ride quality was good.

 

Last week I decided to increase both by 2 psi to see what that would do.  The ride was noticeably harsher, but handling was not affected.  I decided to return the fronts to 34 psi and left the rears at 32 psi for now.  The harshness is gone, so I'll leave it this way (34 F/32 R) for awhile, though I'm thinking I will likely set the rears back to 30 psi.

 

Bottom line: I think the manufacturer's original pressure recommendations are at best a starting point.  Today's tires (radials in particular and even bias-ply reproductions) are not the same as the originals were and certainly roads and the speeds are different.  I agree with others that more pressure is better, in that it will result in lower tire temperature, especially when running with tubes.  Each car is different and it takes some trial-and-error to zero-in on the 'best' pressures for the specific car/tire combination.

[Bloo]

3 hours ago, EmTee said:

The shop manual calls for 23 psi front and 28 psi rear (cold).  I don't really understand why the front pressure would be so much lower, particularly with that big, cast-iron straight-8 up front... 

 

I've seen this on other cars. My guess is it was based on weight, with the car loaded to it's absolute maximum capacity, and of course bias ply tires. It was probably as soft and squishy as they thought they could get away with. The roads were awful. 23 pounds would feel flat to me even on bias ply tires. It would be begging for trouble on radials.

 

 

 

[Akstraw]

On the track, I have used radial chalk or crayon marks a a guide.  Make a few radial marks on each tire, starting up in the tread area and going a couple inches down the sidewall.  Then go riding around for a while, both straightaways and normal driving turns.  Afterward, you can see how much of the tire up or down the sidewall is contacting the road surface.   If there is still chalk on the edge of the tread, you have too much pressure, if the chalk is worn off the sidewall, too little pressure.

[Gronk]

Good advice guys. I'm going to test drive Akstraw's chaulk thing. For what it's worth, (and because I didn't know any better) I have been using the tire pressure recomended for the original bias tires on my 1952 Mercury sedan, for the radials now on it, no problem. I recently drove 125mi mostly highway at speeds up to 65mph, again no problemo! The tires have been on the vehicle at least 20,000 mi. 

[rodneybeauchamp]

28 minutes ago, Gronk said:

Good advice guys. I'm going to test drive Akstraw's chaulk thing. For what it's worth, (and because I didn't know any better) I have been using the tire pressure recomended for the original bias tires on my 1952 Mercury sedan, for the radials now on it, no problem. I recently drove 125mi mostly highway at speeds up to 65mph, again no problemo! The tires have been on the vehicle at least 20,000 mi. 

Pictures please, otherwise it never happened 😀😀😀😀😀😀😀

[Gronk]

 

 

Pictures of what?, the tires? 

 

 

 

 

 

 

 

 

 

 

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[rodneybeauchamp]

On 8/29/2022 at 9:20 AM, Gronk said:

 

 

Pictures of what?, the tires? 

 

Yes, pictures showing the results of your chalk marking at various pressures. That way we understand what you are experimenting with. As they say “A picture is worth a thousand words, several pictures would make it a book or two!”

Rodney 😀😀😀😀😀😀